1. Field of the Invention
The present invention relates to a toner density control method in a developing device used for such image forming apparatuses as electrophotographic reproducing machine and like machines. More specifically, the invention relates to a toner density control method which is capable of precisely controlling the toner density.
2. Description of the Prior Art
An electrophotographic reproducing machine is the apparatus in which an electrically charged photosensitive member (the description hereinafter refers to a photosensitive drum) is exposed to light depending upon the document information to form an electrostatic latent image thereof, the latent image is visualized with toner, and the toner visible image is transferred onto a transfer paper and is fixed. In recent years, the electrophotographic reproducing machines of this kind have been used in every field of industries.
A developing device used for the electrophographic reproducing machines of this kind makes use of a two-component developer which is composed of a toner and a carrier (iron powder). The toner is gradually consumed during developing depending upon the kind and quantity of documents, and the toner density in the developer decreases gradually, which makes it necessary to appropriately replenish the toner. If the toner density is too high, the obtained image density becomes too great and fog often develops.
If the toner density is too low, on the other hand, not only the image density becomes small but also the developer loses durability drastically. For instance, when the toner densities are maintained at 5% and 2%, the durability at the latter density becomes shorter than one-half that at the former density.
Therefore, the mixing ratio of the toner to the carrier must be maintained constant at all times. For this purpose, it has been attempted to detect the toner density in the developer by some means and to so replenish the toner that the detected density value becomes in agreement with a predetermined standard value of density.
There has been proposed a toner density detecting system using inductance to detect the toner density in the developing device that uses a two-component developer. According to this system which utilizes the fact that the carrier included in the developer is a magnetic material, an inductance sensor with coil is disposed in the developing device in order to detect the toner density. Concretely speaking, the toner density is found by measuring the permeability of the developer based on the fact that the mixing ratio of the toner to the carrier varies with a change in the toner density causing the permeability to change.
The output voltage of the inductance sensor is compared with a reference voltage, the toner is so replenished that the output voltage of the inductance sensor will become equal to the reference voltage and, thus, the toner density is controlled to become constant. Such technology has been described in Japanese Patent Publication Nos. 28305/1988 and 5299/1989.
The reference voltage that is used for comparison at the time when the toner is replenished is obtained by throwing a standard developer into the developing device followed by stirring for a predetermined period of time and storing the voltage detected by the inductance sensor in a nonvolatile memory. Generally, the stirring time is set to be, for instance, three minutes, and the density of the standard developer stirred for three minutes is stored in the nonvolatile memory to control the toner density.
In practice, however, the standard developer may have variance to some extent, and the density may not often be stabilized within a predetermined period of time. Moreover, the density may not often be stabilized within a predetermined period of time in the case of the standard developer that was produced a given period of time (several months to one year) ago.
FIG. 4 is a graph showing relationships between the stirring time and the voltage detected by the inductance sensor using a standard developer A just after the production and a standard developer B after a given period of time from the production.
In FIG. 4, the standard developer A represented by solid line is stabilized after the stirring time of about three minutes, but the standard developer B is stabilized for the first time after the stirring time of 10 minutes. In the case of the developer B, if the detected value after the stirring time of three minutes is written onto the nonvolatile memory, the toner density is controlled being deviated from a point of stabilization, and is settled to a density deviated from a proper value. In the case of the developer B of FIG. 4, therefore, the toner density is controlled to become lower than the proper density. Accordingly, the quality of image becomes poor and the life of the developer is shortened.